GCN Circular 45069

C. Y. Dai (NJU), X. Mao, H. N. Yang, C. C. Jin (NAO, CAS) on behalf of the Einstein Probe (EP) team:

We report on the detection of an X-ray transient, designated EP260628b, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient was identified in the telemetry data. The transient started at 2026-06-28T10:04:53 (UTC) and lasted for about 1000 s. The position of the transient is R.A. = 327.606 deg, Dec. = -29.324 deg (J2000), with an uncertainty radius of 2.6 arcmin (90% confidence level, including both statistical and systematic uncertainties). 

The average WXT 0.5-4 keV spectrum can be fitted by an absorbed power-law model with a fixed Galactic hydrogen column density of 2.0 x 10^20 cm^-2 and a photon index of 1.40 +/- 0.46. The derived average unabsorbed 0.5-4 keV flux is 7.18 (-1.80/+2.41) x 10^-11 erg cm^-2 s^-1. The uncertainties are at the 90% confidence level for the above parameters.

EP-FXT performed a follow-up observation of EP260628b starting at 2026-06-29T09:55:12 (UTC), approximately 1 day after the WXT detection. With an exposure time of 1.7 ks, two X-ray sources were detected within the 2.6 arcmin radius of the EP-WXT position.

The first source (EPF_J215025.7-291939) is located at R.A. = 327.6070 deg, Dec. = -29.3276 deg (J2000), with an uncertainty radius of about 10 arcsec (90% C.L., including statistical and systematic uncertainties). The angular separation from the WXT position is 0.28 arcmin. Its 0.5-10 keV spectrum can be fitted by an absorbed power-law model with a fixed Galactic hydrogen column density of 2.0 x 10^20 cm^-2 and a photon index of 3.09 (-0.59/+0.59). The derived unabsorbed 0.5-10 keV flux is 1.61 (-0.30/+0.45) x 10^-13 erg cm^-2 s^-1.

The second source (EPF_J215025.3-291849) is located at R.A. = 327.6054 deg, Dec. = -29.3136 deg (J2000), with an uncertainty radius of about 10 arcsec. The angular separation from the WXT position is 0.56 arcmin. Its 0.5-10 keV spectrum can also be fitted by an absorbed power-law model with a fixed Galactic hydrogen column density of 2.0 x 10^20 cm^-2 and a photon index of 2.61 (-0.55/+0.60). The derived unabsorbed 0.5-10 keV flux is 1.71 (-0.38/+0.62) x 10^-13 erg cm^-2 s^-1. No known X-ray sources are found within either FXT error circle.

Further follow-up observations by EP-FXT will be arranged. Given that the two FXT sources are located close to each other and show comparable X-ray fluxes and similar spectral properties, the association with the WXT transient and the nature of the event remain unclear. Further multi-wavelength follow-up observations are encouraged.

Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).